There are many methods for measuring the vertical displacement of a pile during the installation of the pile. These include those which are attached to the installation equipment and those which are separate from the equipment. Many of these systems have been found to be very effective in the field, but some fail to have the accuracy needed for the “end of drive” portion of the driving process. In that this portion of the drive can determine if the desired depth has been reached, the end of drive measurements need to be accurate, but still cost effective.
More particularly, the desired pile depth is generally known in that the depth of the underlying bedrock is generally known in most areas. Therefore, installers generally know how deep a pile needs to be driven. But, the exact depth or “set point” to achieve a desired bearing parameter is not exactly known. Therefore, exact displacement measurements are more critical to determine when the desired depth (or set point) is reached during the end of drive.
One such method of determining general pile depth involves the use of lines or markings on the side of the pile which can be used to generally determine how deep the pile has been driven. While this is a low cost and effective way to determine the general depth of the pile, the general depth does not determine whether or not the pile has engaged the underlying bedrock and thus the pile has reached its desired set point. Therefore, this method is typically used for estimation of capacity of the pile. As a result, this marking system is typically used with one or more other methods to determine if the set point has been reached.
In order to determine if the set point has been reached, the incremental displacement of the pile can be monitored for each hit by the pile driving hammer. While incremental displacement can be effective to determine the set point, it is difficult to accurately measure for several reasons including, but not limited to, the violent nature of a pile driving hammer blow. For example, this is difficult to measure based on a marker positioned on the ground since the ground moves when the pile is struck by the hammer.
In the United States, incremental displacement measurements can be the number of hammer blows per inch wherein if there are 30-50 blows needed to displace a pile 1 inch, the pile has reached the set point. In other regions, it can be the distance per blow such as how many tenths of a millimeter per blow.
Another system that has been used in the art to monitor and/or record the installation process of a pile is disclosed in Likins U.S. Pat. No. 5,978,749. Likens discloses a pile installation recording system and is incorporated by reference herein and forms part of the specification of this application. More particularly, the Likins Patent discloses a pile installation recording system for both driven piles and auger-cast piles. The disclosed system can record a variety of parameters or data received from one or more sensing devices that are either attached to the pile, attached to the rig, or in a measurement range from the rig and/or the pile. These sensing devices can be any sensing devices known in the art and can be used to determine a wide range of parameters including, but not limited to, sensing the depth of a driven pile.
However, while Likins has been found to be a very effective system in the field, applicant has found that the depth measurement device of Likins has its limitations which for years went unresolved. In this respect and with reference to FIG. 3 of Likins, the sensing device in Likins that is used to determine the depth of the pile and/or auger is joined to line 56 by a three-roller system. While not shown, this three-roller system is supported by the structure of rig 10, but which engages line 56 at or near the boom 12 of rig 10. The three roller system includes an encoder wheel 52. Further, in order to account for the movement of the wire as it is unwound from line real or drum 54, the roller system is joined to a fixed rod (not shown) having a linear bearing configured to allow lateral motion of the measurement system while the wire is unwound. As is known in the art, the wire being unwound from a roll or drum of wire can have significant lateral movement in that the wire is wound across the entire surface of the drum supporting the wire. While the linear bearing arrangement is effective in handling the lateral movement of the wire, it is not effective in handling the changes in load in the direction of line 56 wherein it has been found that the support rod, that is used to support the linear bearing, often deforms or fails in use. In either case, this condition adversely affects the accuracy of the depth sensor and the failure of the system in the field can cause downtime for the pile installation process. Thus, while Likins is an effective system to measuring pile depth, there is a need for a more accurate system for the end of drive portion of the pile driving operation to more accurately determine exactly when the set point is reached.
Yet another problem found with the depth measurement system disclosed in the prior art is that the data produced by the system can be corrupted if the line is allowed to go slack. In that the measurement system is joined to the line, the line itself becomes a variable that must be maintained as a constant. A slack condition in this line affects this constant and can produce error. Moreover, if this slack condition is not noticed, the error in the data may not be known, which would result in a less accurate data set for the pile installation period. Depending on the frequency of the slack condition, and the other factors discussed above, the resulting data could be corrupted enough to prevent its use in evaluating the structural integrity of the installed pile. Again, accuracy levels of this type of system are well suited for general depth measurements, but can be less than ideal for the end of drive measurement of the driving process. Yet even further, the wire measurement system tends to lose accuracy as depth increases in that the stretching of the wire increases. Therefore, a wire measurement system is at its least accurate point during the end of drive.
In addition to Likins, U.S. Pat. No. 6,533,502 to McVay is also incorporated by reference and forms part of this specification. The McVay patent also discloses a system for the determination of pile parameters and includes at least one structure for measuring pile data. The system discloses a structure for measuring pile data that is disposed within a measurement range from a pile wherein a wireless transmitter is communicably connected to the structure for measuring pile data. As with Likins, the wireless transmitter for transmitting the pile data can be in connection with a remotely located receiver.
Another patent incorporated by reference and forming part of this specification is U.S. Pat. No. 6,301,551 to Piscalko. The Piscalko patent discloses applicant's PILE DRIVING ANALYZER® system (PDA™) which obtains, processes and/or stores pile driving data. The PDA system is operable as an independent self-contained unit, or may be used in conjunction with a remote computer system. Position data indicative of the position of a pile, and pile data indicative of characteristics of a pile may be automatically input to the PDA system. When used in conjunction with the remote computer system, the PDA system may be controlled remotely by the remote computer system. Alternatively, the PDA system may be controlled locally by an operator, and data acquired by the PDA system provided to the remote computer system for monitoring and/or storage.
In that it is difficult to accurately measure the exact pile displacement per blow with many of the systems known in the art, these measurements are often calculated by hand. In some situations, by a person marking the blow on graph paper attached to the pile. This method includes an operator holding a pencil to the graph paper and the pencil mark then illustrates the displacement which includes the rebound of the pile after the blow. In that the human body is well adapted to correct for ground movement, this method can be effective for experienced operators. However, and as can be appreciated, this system can vary widely based on the expertise of the operators and is not always a repeatable measure of displacement. Further, while these methods appear simple, they are costly both in the manpower needed to make the measurement and the safety factors that are must be built into these kinds of measurement to ensure that the set point has been reached. Yet even further, it is difficult, costly and/or impossible to quickly digitize this type of data for remote monitoring and/or data storage.
Attempts have been made to address the accuracies relating to depth measurement and the needed accuracy during the end of drive of the pile, but these have been found to have their own set of problems. One such device utilizes lasers to measure pile displacement. However, laser systems have had problems in field since they can only be utilized during clear weather. As can be appreciated, lasers require clear conditions and can be affected by rain and fog. In addition, the laser must be directed in the driving direction of the pile being driven, namely, must be a vertically aimed laser directed toward a horizontal surface for vertically driven piles. In that the only naturally horizontal surface is the one which is being impacted by the pile driving hammer, a ledge of some sort must be added to the pile which results in the measurements being taken from a surface other than a pile surface. As can be appreciated, this adds yet another variable that can affect the accuracy of the system.
Thus, the industry is looking for low cost and accurate ways to determine pile capacity and reaching the set point for driven piles. Further, there is a need in the art for an accurate measurement for this end of drive portion of pile driving that can produce documentable and repeatable measurements that are accurate regardless of the experience of the pile installation crew.